Present simulcast systems require intervention from a central controller to synchronize their timing references at transmitter sites. This typically is accomplished through measurements taken at a plurality of receiver sites and through control instructions sent from the central controller to the transmitter sites. The central controller instructs certain transmitter sites to send measurement signals, and other receiver sites to monitor and determine the reception time of the signals. Subtracting the known propagation time between the sites, the system can compute the time offsets from the sending sites. The central controller then sends instructions to the sending sites for adjusting their timing references to achieve synchronization.
There are several disadvantages with this procedure. Normal user traffic through the transmitters is interrupted during the adjustment process, thereby reducing the overall system capacity. The transmitters and receivers must be instructed by the central controller to listen or receive signals for measurement purposes. This means the transmitters and receivers cannot do adjustments to their timing references without being instructed by the controller. Usually the controller initiates synchronization on some predetermined time rotation basis. If, however, a site has some indication that it has lost synchronization, it must somehow get a message back to the controller and receive instructions before it can resynchronize. The required ongoing central control also requires an additional amount of bandwidth in the links to the sites.
Separate systems that utilize synchronous protocols, such as Motorola's well-known FLEX.TM. protocol, also require synchronization with one another for reasons other than for providing acceptable simulcast transmission. Such systems are required to maintain a common synchronization with one another so that a subscriber unit moving from one system to a next system will not encounter large shifts in synchronization timing, forcing a time-consuming and battery-consuming re-aquisition of synchronization with the next system.
Thus, what is needed is a method and apparatus for synchronizing the transmitters without requiring an interruption of normal user traffic through the system. Preferably, the method and apparatus will operate without the need for ongoing central control intervention, and without an exchange of timing information between a central controller and the transmitters. A method and apparatus for maintaining a common synchronization among multiple separate systems is also needed.